Hel-Fire

It only appears in the deepest slice of the world, far below where the cavern layer begins, and only from enemies that would have left coins behind. That second condition quietly kills the obvious farm: statue spawned enemies never drop money, so no statue setup will ever produce one no matter how long you leave it running. Build a normal platform farm down in the underworld with a water candle and battle potions instead.

This is the yoyo that carries you through the opening stretch of hardmode. The underworld is not touched by the world turning, so you can farm for it before the surface becomes dangerous, which makes it considerably easier to obtain than the Amarok even though the Amarok hits harder. It also throws off light while deployed, so it doubles as a lantern in unlit caves.

The burn it applies is respectable chip damage on its own and it layers with other sources rather than overwriting them. Frost armor adds Frostbite on every connect, a Flask of Cursed Flames adds a third, and across a long boss fight the combined ticks add up to real numbers. Yoyos sit on a target for a while instead of hitting once and leaving, which is exactly why damage over time is worth stacking on this rather than on a sword.